Heat vulcanizable polysiloxane compositions containing asbestos

ABSTRACT

A heat vulcanizable silicone rubber composition comprising 5 to 94.47 percent by weight of an organopolysiloxane polymer, 5 to 75 percent by weight of an asbestos filter, and 0.25 to 10 percent by weight of an acrylic ester. The composition also includes a silica filler, a peroxide curing catalyst and a process aid. The preferred acrylic ester are methyl acrylate and trimethylolpropanethimethacrylate.

United States Patent 1191 Glalster [45] Feb. 6, 1973 [541 HEATVULCANIZABLE 2,546,474 3 1951 Peyret et al. ..260/37 SB ux POLYSILOXA ECOMPOSITIONS 2,860,074 11/1958 Hellund ..260/827 x CO TA ASBESTOS3,577,264 5/1941 Nordstrom... ....260/827X 3,575,910 4/1971 Thomas..260/827 X [75] Inventor: Frank J. Glaister, Ballston Spa,

NY FOREIGN PATENTS OR APPLICATIONS [73] Assignee: General ElectricCompan water 1,360,046 12/1964 France ..260/37 SB ford, N.Y. PrimaryExaminerLewis T. Jacobs Filed: l 15, 1971 Attorney-Donald J. Voss, E.Philip Koltos, Frank L. [21] APP] No: 134,438 Neuhauser, Oscar B.Waddell and Joseph B. Forman [57] ABSTRACT [52] US. Cl. ..260/37 SB,260/465 G 51 Int. Cl ..C08g 51/10, C08g 31/02 A "lcamzable rubber i [58]Field 61 Search ..260/37 $13, 46.5 R, 46.5 G, pr'smg Percent by Weght of260/827 ganopolys1loxane polymer, 5 to 75 percent by we ght of anasbestos filter, and 0.25 to 10 percent by we1ght 5 6] References Citedof an acrylic ester. The composition also includes a silica filler, aperoxide curing catalyst and a process aid. UNITED STATES PATENTS Thepreferred acrylic ester are methyl acrylate and 1trimethylolpropanethimethacrylate. 3,377,311 4/1968 Roch ..260/37 SB3,453,228 7/1969 Metevia et al. ..260/37 SB 9 Claims, N0 Drawings HEATVULCANIZABLE POLYSILOXANE COMPOSITIONS CONTAINING ASBESTOS BACKGROUND OFTHE INVENTION The invention relates to heat-curable polysiloxanecompositions and, in particular, to heat curable polysiloxanecompositions containing asbestos therein.

In prior developments, asbestos has been used as a filler inpolysiloxane compositions which were then cured to produce a siliconeelastomeric composition used for a variety of purposes. In thesedisclosures of the prior art, asbestos was not distinguished from otherinorganic fillers such as calcium carbonate, and not shown to produceany new properties in the resulting composition, but only shown to addstiffness or hardness to the resulting product. However, as mentionedearlier, from the prior art it is seen that asbestos fibers, when usedas a filler in polysiloxane compositions, produced on the whole inferiortypes of silicone rubber as compared to the silicone rubber prepared bythe use of high reinforcing silica fillers. On the other hand, it wasdesired to use asbestos fibers as fillers in polysiloxane compositionsif a polysiloxane rubber composition could be produced with high tensilestrength, high tear strength, high stiffness, which compositions werestill flexible and had desirable per cent of elongation, since asbestosis very cheap.

It was further desirable to obtain a polysiloxane elastomericcomposition which had high heat resistance and a good service life atboth high and low temperatures within the temperature range of minus 40Fup to and above 400F. Thus, in automobiles it is desirable to havegaskets which will have high strength and remain flexible for longperiods of times at temperatures as high as 400F. On the other hand, itis also desirable that gaskets which often act as seals do not becomebrittle at low temperatures such as 40F to which the mechanisms of theautomobile may be exposed. It is also desirable in this respect thathoses formed from a polysiloxane composition have a high tensilestrength and which retain their strength at high temperatures, as wellas at very low temperatures.

Another use for elastomers which have high heat resistance, as well asremain flexible at low temperatures, is to cover electrical wires andelectrical components. Since such electrical wires and electricalcomponents are normally exposed to very high temperatures, as well aslow temperatures, it is required that the material covering andprotecting the wires does not degrade in strength or become brittle athigh or low operating temperatures. It is also desired to havepolysiloxane elastomers with high tensile strength and stiffness orhardness, high tear strength, good abrasion resistance, flexibility andthe desired elongation so that such material could be used as a coveringmaterial. The present polysiloxane elastomeric compositions aresometimes lacking in sufficiently high tensile strength and hardness. Toproduce such polysiloxane elastomeric composition having the aboveproperties which could be put to the uses mentioned above, it wassuggested that different fillers be used or incorporated into thepolysiloxane composition so that the resulting composition will have thedesired properties. However, all approaches or uses of all fillers up tothe present time have been found lacking in some respect or other. Inother words, that is, although with the use of certain types of fillers,such as high reinforcing silica fillers in certain polysiloxanecompositions, were superior in properties to other known polysiloxanecompositions,

these superior polysiloxane compositions were still not 5 found to meetall the performance requirements or found not to have as highperformance values as would be desired.

As mentioned previously, asbestos was considered for use as a filler inpolysiloxane compositions. However, even with the use of asbestos as afiller, the elastomeric polysiloxane compositions that were obtainedwere found to have only low tensile strength, low tear resistance, aswell as not to have a high hardness.

It is one object of the present invention to produce a heat-curablepolysiloxane composition with high tensile strength and exceptionallyhigh heat resistance at high temperatures.

It is anothe'r object of the present invention to produce a heat-curablepolysiloxane composition which has good flexibility at low temperatures,as well as at high temperatures.

It is yet still another object of the present invention to produce aheat-curable polysiloxane composition having asbestos fibers therein,and having a high hardness value as well as the desired elongation.

It is yet another aim of the present invention to provide a process forproducing a heat-curable polysiloxane composition which has a hightensile strength, exceptionally good heat resistance, the desiredflexibility within a broad temperature range, a high hardness and thedesired per cent of elongation.

These and other objects of the present invention are accomplished inaccordance with the polysiloxane composition and the process forproducing this polysiloxane composition set forth below.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a heat-vulcanizable silicone rubber composition comprisingto 94.75 percent by weight of an organopolysiloxane polymer having aviscosity of at least 100,000 centipoise at 25C of the formula,

)u 4-ul2 (I) 5 to 75 percent by weight of asbestos fiber and 0.25 to10.0 percent by weight of an acrylic ester selected from the groupconsisting of when E represents the radical,

R is a radical selected from the group consisting of monovalenthydrocarbon radicals, halogenated monovalent hydrocarbon radicals andcyanoalkyl radicals, R is a radical selected from hydrogen and the sameradical as R, R is a divalent hydrocarbon radical, a varies from 1.95 to2.01, inclusive, n is a whole number that varies from 2 to 4 and xvaries from 1 to 10. There is also included in the above composition aperoxide curing catalyst which comprises 0.1 to 8.0 percent by weight ofthe composition. There may also be added to this composition to 60percent by weight of the organopolysiloxane of silica filler, as areinforcing agent. However, this silica filler is not necessarily a partof the composition. Within the weight per cent of the organopolysiloxaneshown above there may also be included I to 25 percent by weight of theorganopolysiloxane of a process aid. Within the scope of the presentinvention there is the process of mixing the ingredients set forth aboveand heating the resulting mixture to a temperature in the range of 80Cto 650C to cure it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The radical Rrepresents hydrocarbon radicals, such as aryl radicals and halogenatedaryl radicals such as phenyl, chlorophenyl, xylyl, tolyl, etc; aralkylradicals, such as phenylethyl, benzyl, etc; aliphatic, haloaliphatic andcycloaliphatic, such as alkyl, alkenyl, cycloalkyl, haloalkyl, includingmethyl, ethyl, propyl, chlorobutyl, cyclohexyl, etc; cyanoalkylradicals, such as cyanoethyl, cyanopropyl, cyanobutyl, etc. The radicalsrepresented by R are selected from the same radicals as represented byR, or hydrogen. The radicals represented by R are alkylene and aryleneradicals of two to carbon atoms, such as methylene, ethylene, etc.Preferably, the radicals R and R have eight carbon atoms or less. Of theR radicals in formula (I), at least 50 percent of the R groups arealltyl radicals and preferably methyl radicals. In addition, at least0.1 percent by weight of the R groups in formula (I) are vinyl orrepresent another unsaturated alkenyl radical. However, the vinylradical is preferred to the other alkenyl radicals. Thus, in order toobtain the preferred elastomeric composition of the present case whichhas the exceptional physical properties desired, it is preferred thatthere be a minimum of at least 0.1 percent by weight of vinyl or otheralkenyl radicals attached to the silicon atom. The concentration of thevinyl or other alkenyl radicals in formula (I) may vary within the rangeof 0.1 to 0.6 percent by weight of the polysiloxane polymer. The Rradicals in theformulas of the acrylic esters as set forth above can beany of the groups specified above. However, preferably the radicals inthe acrylic ester formulas is a lower alkyl having less than eightcarbon atoms and is preferably methyl. Besides the organopolysiloxane,there are two necessary ingredients in the composition of'the presentinvention, that is, the asbestos fiber and the acrylic esters. Theasbestos fibers are classified in six different categories in accordancewith their basic formulas which six categories are given the name oftremolite, crystotile, crocidolite anthophyllite, amosite andactinolite. All of these types of asbestos fibers have been found to beuseful in the composition of the present invention. The crystotile havebeen found to be superior in producing elastomeric compositions of highphysical properties as compared to the other types of asbestos fiber. Inparticular, crystotile and crocidolite asbestos fiber produceelastomeric compositions of high tensile strength, while anthophylliteasbestos fibers produce an elastomeric composition with exceptionallyhigh heat resistance. As a matter of fact, the tensile strength ofelastomeric compositions prepared having an anthophyllite asbestos fibertherein actually improves upon heat aging. For further information as toasbestos, reference is made to the article on Asbestos by G. P. Jenkinsappearing in Industrial Minerals and Rocks (Seeley W. Mudd Series),published by the American Institute of Mining, Metallurgical andPetroleum Engineers (1960). Within the general formulas of acrylicesters given above, the most preferred acrylic esters are:

To produce acrylic esters, ethylene chlorohydrin is reacted with sodiumcyanide at room temperature to produce ethylene cyanohydrin. Theethylene cyanohydrin is then reacted with methanol in the presence of0.1 to 5 percent by weight of sulfuric acid at elevated temperatures inthe range of to I50C to produce methyl acrylate. The resulting methylacrylate may then be reacted with any saturated alcohol so that theorganic group in the alcohol substitutes itself for the methyl grouplocated on the carboxy group in the well known alcoholysis type ofreaction.

In the case where R is other than hydrogen, such as methyl, an organiccompound of the formula,

is reacted with sodium cyanide at room temperature to produce theresulting cyanide product. This cyanide product is then reacted withmethanol in the process of 0.1 to 5.0 percent by weight of H 50 at roomtemperature to produce methylacrylate. If desired, this may be reactedwith various alcohols of the formula,

The methyl acrylate is one of the preferred acrylic esters used in thepresent invention. However, it can also be used as an intermediate inproducing other acrylic esters which come within the scope of theformulas set forth.

This methyl acrylate can be used to produce the other acrylic esters byalcoholysis. Thus, the methyl acrylate can be reacted with alcohols ofthe above formulas. If these alcohols are reacted with the methylacrylate in a normal alcoholysis reaction and using the proper molarproportions of the alcohol versus the methyl acrylate, the methyl groupwill be substituted by the alcohol group.

The alcoholysis reaction involves mixing the two reactants in the propermolar proportions at elevated temperatures at a range of 50 to 100C with0.1 to 2 percent by weight of acid catalyst and allowing the mixture toremain at the elevated temperature for a period of 1 to hours. Duringthe reaction, the methanol that is formed is boiled off leaving thedesired product. The catalyst which may be used is a strong acid, suchas hydrochloric, sulfuric or nitric acids or, if desired,paratoluenesulfonic acid. In the alcohol formulas shown above, R and Rare as defined previously.

A more detailed presentation of forming the acrylic esters of thepresent case is to be found in the publication Acrylic Esters, byReinhold Publishing Company, 1954. in a mixture of organopolysiloxanepolymer of formula (1), asbestos fiber and acrylic ester, there isgenerally in the mixture of to 94.75 percent by weight of thepolysiloxane, and preferably 50 to 94.75 percent by weight. In thismixture, there is 5 to 75 percent of asbestos filler, preferably theconcentration of the asbestos fiber varies from to 60 percent by weightof the mixture. As little as 5 percent asbestos fiber would have someeffect on the resulting properties of the polysiloxane. At 25 percentasbestos fiber in the polysiloxane mixture there is a substantial effecton the desired physical property of the resulting composition. When theconcentration of the asbestos fiber exceeds 75 percent by weight, thereis too much asbestos fiber relative to organopolysiloxane and, as aresult, the composition has poor physical properties. Thus, a preferableconcentration of IS to 60 percent by weight of asbestos fiber providesan organopolysiloxane with the best physical properties. With respect tothe acrylic esters, there may be as little as 0.25 percent by weight ofone of the acrylic esters defined above in the polysiloxane compositionand this amount of acrylic ester will produce some resulting benefit inthe physical properties of the resulting cured silicone rubber. Itshould be understood that the less asbestos fiber there is in thepolysiloxane composition, the less acrylic ester is needed. Further,when there is as much as 75 percent by weight of asbestos fiber in thecomposition, there maybe as much as 10 percent by weight of acrylicester, based on the weight of the resulting composition. If there ismore than 10 percent by weight of acrylic ester. in the composition,such an excess is not needed and does not produce any desirable results.Preferably, the amount of asbestos fiber is between 25 to percent of thecomposition. The preferable concentration for the acrylic ester is 0.255 percent by weight.

There are also within the scope of formula (1) polydiorganosiloxaneswhich can be copolymers containing two or more differentdiorganosiloxane units therein and copolymers of dimethylsiloxane unitsand methylphenylsiloxane units; or copolymers of methylphenylsiloxaneunits, diphenylsiloxane units, dimethylsiloxane units andmethylvinylsiloxane units, as well as copolymers of dimethylsiloxaneunits, methylvinylsiloxane units and diphenylsiloxane units.

Preparation of the diorganopolysiloxane of formula (1) which can containboth saturated and olefinically unsaturated hydrocarbon groups may becarried out by any of the procedures well known to those skilled in theart. Such polysiloxanes can be produced by following a procedureinvolving hydrolysis of one or more hydrocarbon-substituteddichlorosilanes in which the substituents consist of saturatedhydrocarbon groups to produce a crude hydrolyzate containing a mixtureof linear and cyclic polysiloxanes. Further, one or morehydrocarbon-substituted dichlorosilanes whose hydrocarbon substituentscomprise one or more olefinically unsaturated hydrocarbon groups arehydrolyzed to produce a crude hydrolyzate containing a mixture of linearand cyclic polysiloxanes. The two crude hydrolyzates are depolymerizedby being treated with KOH to form mixtures of low boiling, low molecularweight cyclic polymers and undesirable material such as themonofunctional and trifunctional chlorosilane starting material. Theresulting compositions are fractionally distilled and there is collectedtwo pure products containing the low boiling, low molecular weightcyclic polymers free of any significant amount of monofunctional andtrifunctional groups.

In order to depolymerize the crude hydrolyzates, there is added to thema catalyst and the mixture is heated at a temperature above 150C toproduce and recover by evaporation a product consisting of low molecularweight cyclic polysiloxanes comprising, for example, about percent ofthe tetramer and 15 percent of the mixed trimer and pentamer. When thehydrocarbons on the silicon atom are methyl, materials resulting fromthe presence of monomethyltrichlorosilane in this initial productproduced from dimethyldichlorosilane remain as residue in thedistillation vessel or tower.

The distillate, consisting essentially of low molecular weight cyclicorgano polymers, free of any significant amount of monofunctional andtrifunctional groups, is collected in a vessel. The then dried cyclicsiloxane contains less than 50 ppm of water. The cyclic dimethyl,methylvinyl and diphenyl cyclic siloxanes are prepared in the same way.

The pure cyclic siloxanes are added in the desired proportions in areaction vessel so as to be subjected to an equilibration reaction toform the polysiloxanes of formula (1). Thus, about 1.5 17 mole per centcyclic diphenylsiloxane can be added to 82 97.5 mole per cent dimethylcyclic siloxanes. Then 0.1 l-O mole per cent of methylvinyl cyclicsiloxane may be mixed with the dimethyl and diphenyl cyclic siloxane orother desired proportions of the cyclic siloxanes may be used to producethe desired polymer. To the above mixture of pure cyclic siloxanes thereis added a polymerization catalyst, such as KOH. The KOH breaks the ringof cyclic siloxanes to form a potassium silanolate which has theformula,

This compound, the potassium silanolate, thus can attack other cyclicsto break the rings and increase the chain length of the siloxanesformed. There is further added to the reaction mixture an amount of oneor more monofunctional compounds calculated to function as end-blockersfor limiting the degree of polymerization and consequently the lengthsand molecular weights of the linear polysiloxane chains, and forstabilizing the polymers. Usually, a small amount of monofunctionalcompounds are added to function as end-blockers so as to regulate thechain length of the polymers. Preferably, a compound is used as thechain-stopped groups having the formula,

Other monofunctional compounds that may be employed satisfactorily forcontrolling polymer growth include, among others, hexamethyldisiloxane,tetramethyldiethoxydisiloxane, diethyltetraethoxydisiloxane anddivinyltetraethoxydisiloxane.

The equilibration reaction is carried out from 2 to 4 hours until about85 per cent of the cyclic diorganosiloxanes have been converted topolymers endstopped with monofunctional groups. When the 85 percentconversion point has been reached, there are just as many polymers beingconverted to the cyclic siloxanes as there are cyclic siloxanes beingconverted to the polymer. At that time, there is added to the mixture asufficient amount of an acid donor, such as phosphorous acid, that willneutralize the KOH catalyst so as to terminate the polymerizationreaction. The cyclic diorganosiloxanes in the reaction mixture are thendistilled off to leave the polydiorganosiloxane gum which is useful inthe'present invention.

Alternatively, the mixture of polydiorganosiloxane may be then furtherreacted with the cyclic siloxanes therein and then during compounding ofthe mixture with process aid and fillers on a doughmixer, the

remaining cyclic siloxanes may be removed by a gas purge.

The polydiorganosiloxane is produced so that it preferably registers apenetration of 50 4,000 mm per minute on a standard penetrameter.Further, the polymer preferably has a molecular weight in the range of100,000 to 2,000,000 and a viscosity of l00,000 to 100,000,000centipoise at 25C.

Hydrocarbon-substituted polysiloxanes whose pendant groups consistlargely of groups other than methyl, such as ethyl or other saturatedhydrocarbon groups and olefinically unsaturated hydrocarbon groups otherthan, or in addition to, vinyl groups can be produced by means ofprocedures similar to that described above or by means of proceduresmodified in accordance with the known characteristics of the varioushydrocarbon groups to be included.

The polydiorganosiloxane gum employed is preferably produced underconditions so controlled as to avoid the incorporation therein of anysignificant amounts of trifunctional compounds, groups, or molecules toavoid crosslinking of linear polysiloxane chains through silicon andoxygen atoms and the incorporation therein of any significant amount ofmonofunctional compounds or radicals than those specifically provided toserve as end-blockers for limiting the degree of polymerization.Accordingly, the starting polydiorganosiloxane gum contains 2.0hydrocarbon groups per silicon atom. Deviations from a ratio of2 to 1,for example, ratios of 1.95 to 2.01 will be insignificant for allpractical purposes since it will attribute to the presence of otherhydrocarbon groups whose total numbers will be insignificant as comparedwith the total number of hydrocarbon groups attached to silicon atomsoflinear polysiloxane chains.

In producing the silicone rubber composition of the present invention,there is utilized any of the filler materials of the highly reinforcingtypes consisting of inorganic compounds or any suitable combination ofsuch filler materials employed in the production of elastomers as iscustomary in the prior art. There is preferably employed finely dividedsilica base fillers of the highly reinforcing type which arecharacterized by a particle diameter of less than 500 millimicrons andby surface areas of greater than 50 square meters per gram. Inorganicfiller materials of a composition other than those preferred can beemployed alone or in combination with the preferred fillers with goodresults. Such filler materials as titanium dioxide, iron oxide, aluminumoxide, as well as the inorganic filler materials known as insert fillerswhich can include among others, diatomaceious earth, calcium carbonateand quartz can preferably be employed in combination withhighlyreinforcing silica fillers to improve the tensile strength or thehardness of the elastomeric product. Other examples of suitable fillersare diatomaceous silica, aluminum silicate, zinc oxide, zirconiumsilicate, barium sulfate zinc sulfide, aluminum silicate and finelydivided silica having surface-bonded alkoxy groups.

There is preferably employed in the present compositions 10 percent byweight of said polysilox ane gum of the inorganic filler and preferably20 to 60 percent by weight.

There is also employed in the present composition 1 to 25 percent andpreferably 5 to 15 percent by weight based on the polydiorganosiloxanegum of a process aid for preventing the gum and the filler mixture fromstructuring prior to curing and after compounding. One example of such aprocess aid is a compound of the formula,

where R is a member selected from the class consisting of methyl andphenyl, X is a member selected from the class consisting of OH, --NH, orOR, where R is methyl or ethyl, n has a value of from 2 to 4, inclusive,and b is a whole number equal to from to 10, inclusive. Further detailsas to the properties, as well as the method of preparation of thecompound of formula (3), are to be found in the disclosure of MartellockU.S. Pat. No. 3,464,945 which is herein incorporated by reference.

The process aid may also be a dihydrocarbon-substituted polysiloxane oilhaving a hydrocarbon substituent to silicon atom ratio of from 1.6 to2.0 and whose hydrocarbon substituents comprise at least one memberselected from the class consisting of methyl, ethyl, vinyl, allyl,cyclohexenyl and phenyl groups, said polysiloxane oil comprisingpolysiloxane molecules containing an average of from one to two loweralkoxy groups bonded to each of the terminal silicon atoms where thealkoxy groups are selected from the class consisting of methoxy, ethoxy,propoxy and butoxy.

Preparation of the alkoxy-containing hydrocarbonsubstituted polysiloxaneoils that can be employed as a process aid in the present invention canbe carried out by producing one or more types of cyclicdihydrocarbon-substituted polysiloxanes from one or more types ofdihydrocarbon-substituted dichlorosilanes and dialkoxysilanes inaccordance with the hydrolysis, depolymerization and fractionaldistillation procedures described in detail above with reference to thepreparation of the gum of formula l Then one or more types of cyclicsiloxanes so produced are mixed with predetermined amounts of adihydrocarbon-substituted dialkoxysilane and the mixture is subjected toan equilibration treatment under controlled conditions to produce thedesired alkoxy end-blocked hydrocarbonsubstituted linear polysiloxaneoil.

The alkoxy-containing hydrocarbon-substituted polysiloxane oils suitablefor use in the present invention are relatively low molecular weightpolysiloxane oils whose polymer chains have at least four and as much as35 and more dihydrocarbon siloxy units per molecule. The polysiloxaneoils preferably have an average of at least one and not more than twoalkoxy groups bonded to each of the terminal silicon atoms of themolecule. A more detailed disclosure of the alkoxy end-blockedpolysiloxane process aids, as well as their method of preparation,- isto be found in the disclosure of Fekete, U.S. Pat. No. 2,954,357 whichis hereby incorporated into this specification by reference.

There may also be used as a process aid hydroxylated organosilanes whichcontain from one silicon-bonded hydroxyl per 70 silicon atoms to twosilicon-bonded hydroxyls per silicon atom and contains from 1.9 to 2.1hydrocarbon radicals per silicon atom. The remaining valences of thesilicon atom are satisfied by oxygen atoms. The hydroxylated materialsinclude both The hydroxylated siloxanes may be prepared by any suitablemethod, such as heating said siloxanes with steam under pressure attemperatures of about C or hydrolyzing silanes of the formula R,,SiX.,where X is any hydrolyzable group such as Cl, OR, H, -OOR and R is amonovalent hydrocarbon radical. The former method is preferred for thepreparation of those hydroxylated materials in which the hydrocarbonradicals are alkyl, while the latter method is best for the siloxanes inwhich hydrocarbon radicals are monocyclicaryl hydrocarbon radicals.Further, detailed information as to the hydroxylated organosiloxaneswhich may be used as process aids is to be found in Konkle et al U.S.Pat. No. 2,890,188, the disclosure of which is being incorporated intothis application by reference.

Any of the above process aids may be used alone or mixtures thereof maybe used in the above-defined concentrations. Further, other suitableprocess aids may also be used in the silicone rubber compositions of thepresent invention.

The curing of the silicone rubber composition of the present inventioncan be effected by chemical vulcanizing agents or by high energyelectron radiation. More often, chemical vulcanizing agents are employedfor the curing operation and any of the conventional curing agents canbe employed. The preferred curing agents are organic peroxidesconventionally used to cure silicone elastomers. Especially suitable arethe dimethyl peroxides which may have the structural formulas,

I R R R wherein R represents the same alkyl group throughout, or alkylgroups of two or more different types and n is zero or a larger integer.

Among the specific peroxide curing catalysts that are preferred aredi-tertiary-butyl peroxide, tertiary-butyltriethylmethyl peroxide,tertiary-butyl-tertiary-butyltertiary-triphenyl peroxide and di-tertiaryalkyl peroxide such as dicumyl peroxide. Other suitable peroxidecatalysts which effect curing through saturated as well as unsaturatedhydrocarbon groups on the silicon chain are aryl peroxides which includebenzoyl peroxides, mixed alkyl-aryl peroxides which includetertiary-butyl perbenzoate, chloroalkyl peroxides such as 1,4-dichlorobenzoyl peroxide; 2,4-dichlorobenzoyl peroxide,monochlorobenzoyl peroxide, benzoyl peroxide, etc. Generall, 0.1 8percent of said peroxide, by weight of the polydiorganosiloxane gum isused to cure the silicone rubber composition and preferably 0.5 3.0percent by weight.

It is found that the composition of the present case after it has beencured into a calendered sheet, can be spread to 30 percent of itslongitudinal length before it breaks. However, it can be stretched to 50percent of its transversal width before it breaks. Unexpectedly, the useof the anthophyllite asbestos fiber in the composition of the presentinvention produces unexpected results in this area. For instance, theuse of the anthophyllite asbestos fiber in the composition of thepresent invention after the composition has been cured to the sheet,allows the calendered sheet to be stretched along its longitudinaldirection to 50 percent elongation before it breaks. However, the samesheet can be stretched to 250 percent elongation in the transversedirection before it breaks. Thus, it can be seen that the use ofasbestos fibers allow calendered and cured silicone elastomeric sheetsto be stretched more in the transverse direction as compared to thelongitudinal direction before the sheet is ruptured.

There also can be incorporated into the present silicone rubbercomposition, pigments such as titanium dioxide which may be incorporatedinto the composition at a concentration of 0.05 to 3 percent by weightof the organopolysiloxane. Titanium dioxide is incorporated as a pigmentinto the composition of the present case in order to make the curedsilicone elastomeric sheets prepared therefrom impervious to light.There may also be incorporated heat stabilizers, such as iron oxides,carbon black, rare earth octoates, urethanes, etc.

There is preferred as a heat stabilizer to be incorporated into thecomposition 0.] to 5 percent by weight of the organopolysiloxane of Fe oIn the practice of the invention, the present polysiloxane compositionis produced by mixing the organopolysiloxane polymer, the silica orother types of filler and the process aid. As this mixture is formed,then the asbestos fiber, the acrylic ester of the present case and theperoxide curing catalyst are mixed into the composition. At this pointthere may be added the iron oxide or a pigment, such as the titaniumdioxide. The order of addition of the latter ingredients is notcritical, it is only important that the organopolysiloxane gum, thefiller and the process aid be mixed together first before the otheringredients are added. The other ingredients, such as the peroxidecuring catalysts and asbestos fiber and acrylic ester, as well as thetitanium dioxide and iron compound, may then be added in whatever orderis desired. The various ingredients in the mixture can be blendedtogether by use of standard rubber mixing equipment, such as doughmixer,rubber mill, waring blender and the like. One procedure, for example, isto add the inorganic filler to the polymer gum while it is being milled,

followed by the addition of the process aid and then adding the asbestosfiber, acrylic resin, peroxide curing catalyst and the other additionalingredients desired. Another procedure that can be employed is todoughmix the polymer and the inorganic filler, the process aid and theperoxide curing catalyst while it is being milled on the rubber mill andthen adding the other ingredients thereafter. Those skilled in the artwould know by the properties desired in the cured product and theapplications to which the cured product is to be employed, and thenature and amount of the particular ingredients utilized, the manner ofblending to produce the desired organopolysiloxane composition. To formthe organopolysiloxane, the polymer, inorganic filler and process aidwhich is optional, are added in a doughmixer and after the mixture iscomplete, the mixture is taken and put on a mill. While it is on themill there is added to the mixture the peroxide curing catalyst, theacrylic ester and the asbestos fiber in any desired order. The milledsheets are then cured in a manner well known in the art. Theorganopolysiloxane composition can be converted to the cured product byheating at temperatures in the range of to 650C, depending upon thenature of the curing catalyst, duration of cure, amount and type offiller, etc., as well as the amount of the other ingredients. The directconversion of the polysiloxane composition to the cured product can beeffected as a result of the conditions normally utilized duringconventional molding, extrusion and calendering operations. For example,depending upon the curing catalyst used, the temperature from 80 to 300Ccan be employed for compression and transfer molding for either 30minutes or more or one minute or less.

Hot air curing at the temperatures of from to 650C or steamvulcanization at temperatures from to 210C can be employed from periodsfrom 5 to 10 minutes, or a matter of seconds. The sheets can becalendered or milled first and then press-cured at 200 400C for 30seconds to 10 minutes or passed into an oven where they can be airheated to a desired temperature range of l00 to 300C.

in order that those skilled in the art will be able to understand thepractice of the present invention, the

following examples are given by way of illustration and not by way oflimitation. All parts are by weight.

Example 1 The high tensile strength, the high tear strength, excellentheat resistance of the compositions of the present case are especiallygood in the case where crysotile asbestos fibers are used in thecomposition of the present case, as illustrated by this example. Thereis mixed with l00 parts of polysiloxane polymer having the formula,

up... Li...

15 parts of a process aid which is methoxy-stopped and having twelvediphenyl-, dimethyl-, methylphenylsiloxy groups therein and 50 parts ofsilica filler to form Composition A. The above ingredients are mixed ina dough-mixer. Afterwards samples of Composition A are placed on a milland there is milled per 100 parts of Composition A various amounts oftrimethylolpropenetrimethacrylate and dicumyl peroxide. The resulting.mixtures are milled into sheets which are then press-cured at atemperature of 330F for 1 hour. After the curing period has passed, thecured rubbers are subjected to different physical tests to determine thephysical properties. Different samples of the cured polysiloxane rubbersheets are then heat-aged for different periods of time and thedifferent samples which are heat-aged for different periods of time arethen tested to determine their physical properties. Furthermore, adifferent mixture of Composition A with the curing catalyst and adifferent amount of asbestos are prepared and evaluated. The results ofthese tests are given below in Table I.

Ingredients Mixture A Mixture B Composition A I I00 Crysotile Asbestos50 I00 Trimethylol ropanc- I 24 24 24 trimethacry ate L 1.5 hour/ hours/hours/ hours/ Dicumyl Peroxide L0 1.0 Physical Properties 350F 300F 350F400F TABLEI 5 Initial, Cured I5 Minutes at 300 F T '1 5 th ogo 92 19501880 Tensile strength 1560 850 eiii' inen fie reefii 50 40 40 40Eiongflllo" 40 40 Durometer 86 86 87 88 Hflrdnqss 87 90 Tear ResistancePPI 280 280 290 I80 am B 200 95 I68 hrs/ res hrs 168 hrs Resilience,Bashore. 40 25 300F /35()F /4()()F Heat Aged, 18 Hours at 350 F TensileStrength I700 I250 Elongation 25 Hardness 90 90 Tear Die B 255 175Tensile Strength psi I850 I350 840 Heat Aged. 96 Hours at 350 F 5Elongation percent 40 40 40 Tensile Strength I830 Durometer 86 86 85Elongation Tear Resistance PPI I85 I85 150 Hardness 90 T D' B 245 Aged,1 g H 350p The silicone rubber elastomer composition has good TlcnsileStrength 226(5) physical properties sufficient for the uses enumeratedgs 'g g g g 20 above to which the composition of the present case TearDie? F 250 may be put to such use as gaskets and radiator hoses Heat Age96 Hours at 400 Tensile Strength 950 and other such uses. Elongation I5Hardness 90 Example 3 Tear I50 25 Although crysotrle asbestos fiberswere found to rm- From these results, it is seen that Mixture Ainitially WP the Slhcohe rubber composmoh yields a composition with ahigher tensile strength, as tehshe Strength than 9 types of asbestos h hwe as a higher percent of elongation than Mixture B was found thatcrocrdolrte asbestos fibers will yield thus it is desirable to have thepresent composition at 3 polyslloxane compositions wrth better heatresistance. 50 parts rather than 100 parts of crysome asbestos fiber Toillustrate the advantage of usmgerocrdolrte asbestos per 100 parts ofComposition It is noted that with fibers in the polysiloxane compositionof the present both Mixture A and Mixture B which are heat-aged at h?mxture was prephred h fibers and 350F for various periods of time, thetensile strength posmoh A of Example 1 m the fohoWmg amounts tends toincrease as well as the tear strength and hardness. On the other hand,during the heat-aging the eIongfigg gyl gb s gation decreases to someextent. Thus, it can be seen Crocidolite Asbestos Fibers 50.0 from thepresent results that the composition of the gamg g g I 5 present casehas excellent heat resistance at tempera- B prgroxide tures up to 350and 400F. 40

E l 2 After both the ingredients were milled together and mm? c curedinto a sheet, the sheets were then placed in an An organopolysiloxane,filler and process aid mixoven and cured at 350F for 2 hours. Samplesfrom the ture was prepared with the same ingredients and at the CuredSheets were ta e and heat-aged for various same concentrations asComposition A in Example I, RerlOdS f e 80 as to determine the physicalproperwhich will be referred to herein as Composition A. Into ties afterheat'agmg- The results are shown Table 100 parts of Composition A therewas milled the folbelow- Iowing ingredients:

T Ingredients Parts ABLE Composition A 100.0 Chrysotile Asbestos Fibers50.0 physlcal Propelties g g Fe,O, Masterbatch 2.0 Methylacrylate 1.5Dicumyl Peroxide 2.0

Physical 24 hrs/ 24 hrs/ 24 hrs/ 24 hr The mixture was milled into asheet, was then heat Properties Initial 35osF 0 480; i} cured at 350 Ffor a period of 1 hour. After that, various samples were heat-aged fordifferent periods of time. At the end of the heat-aging period, thesheets T ensl e were then tested for the different physical propertiesto swans"! 1200 I440 I000 I050 040 yield the results shown in Table IIbelow. Elongation 50 40 40 40 25 Durometer 84 86 86 86 88 ll-zlexibilityGood Good Good Good Good ear TABLE I! SDtrergth, 220 275 195 230 I6Physical Properties at Various Heat Aging 5 Temperatures and Times 7days/ 7 days/ 7 days/ 7 days/ Initial 350F 400F 600F [t is seen fromTable 111 that the cured elastomeric composition has good heatresistance properties at a temperature range of 300 to 400F. It can beappreciated from the data given in Table 111 that the crocidoliteasbestos fibers do not increase the initial tensile strength of thecured polysiloxane composition of the present case as high as whencrysolite asbestos fibers are used. However, the tensile strength, aswell as the per cent elongation and tear strength do not degrade as muchduring heat-aging at temperatures as high as 350F, to 400F and even atthe extreme temperatures of 600F when crocidolite asbestos fiber isused. Thus, it is indicated by the data obtained above that if thepolysiloxane elastomeric composition is to be used at an excessivelyhigh temperature, that is at about 350 or 400F for long periods of time,then it would be highly advantageous to use crocidolite asbestos fibersin the composition of the present case. It is shown by the data thateven after the silicone composition had been heat aged for'7 days at600F, it still had good physical properties and thus could stillfunction for the use for which it is intended, such as high temperaturegasket seals, cooling hoses or coverings for electric wires andcomponents.

Example 4 Anthophyllite asbestos fiber, as distinguished from the othertypes of asbestos fibers, has the unique property that upon heat-aging,the tensile strength of the cured elastomeric composition increases. To100 parts of Composition A of Example there were added the followingingredients:

INGREDIENTS Parts Composition A 100 Antho hyllite Asbestos Fibers 50Methy acrylate 1.5 Dicumyl Peroxide 2.0

The resulting mixture of the above ingredients was milled on the milland the sheet that was formed was then placed in the mold andpress-cured for five As indicated from the results above, whenanthophyllite asbestos fibers are used, the tensile strength uponheat-aging improves while the durometer remains substantially the same.

Example 5 Using a polysiloxane, filler, and process aid blend preparedin accordance with Composition A of Example 1, there was preparedseveral samples of the composition to which samples, there were addedvarious ingredients. After the samples were milled into sheets, thesheets were cured in a mold for 5 minutes at 325F. The cured sheets werethen tested to determine their physical properties which are given inTable V below.

The results above amply show the advantageous increase in physicalproperties where the acrylic resin is used with the asbestos fiber toprepare a heat cured silicone rubber composition.

lclaim:

1. A heat vulcanizable silicone rubber composition comprising (a) 15 to94.75 percent by weight of an organopolysiloxane polymer having aviscosity of at least 100,000 centipoise at 25C of the formula,

)(I 4-uI2 (b) 5 to percent by weight of asbestos fiber, and (c) 0.25. to10.0 percent by weight of an acrylic ester selected from the groupconsisting of oiii -s E-R, E-R E, E(C,.Hz..0),R=o-E, R- H -E CHr-E andGE -E E-CH2CCHz*-E CHa-E where E represents the radical where at least50 percent by weight of the R radicals in Formula (1) are alkyl and atleast 0.1 percent by weight of the R radicals in Formula (1 are alkenyland the remaining R radicals in Formula (1) may be selected from theclass consisting of aryl, arlakyl, cycloalkyl and cyanoalkyl, where theR radicals in the acrylic ester formulas are lower alkyl, where R isselected from the class consisting of alkyl, aryl, alkenyl,

cycloalkyl and hydrogen, R is a divalent hydrocarbon radical and avaries from 1.95 to 2.01, inclusive, n is a whole number that variesfrom 2 to 4 and x varies from 1 to 10.

2. The composition of claim 1 further including a silica filler whichcomprises to 60 percent by weight of the organopolysiloxane.

3. The composition of claim 2 further including 0.1 to 8 percent byweight of the organopolysiloxane of a peroxide curing catalyst.

4. The composition of claim 1 wherein there is added to theorganopolysiloxane a process aid which comprises 1 to 25 percent byweight of said organopolysiloxane.

5. The composition of claim 4 wherein the process aid is adihydrocarbon-substituted polysiloxane oil having a hydrocarbonsubstituent to silicon atom ratio of from 1.6 to 2.0 and where saidhydrocarbon substituents comprise at least one member selected from theclass consisting of methyl, ethyl, vinyl, allyl, cyclohexenyl and phenylgroups, said polysiloxane oil comprising polysiloxane moleculescontaining an average of from one to two lower alkoxy groups bonded toeach of the terminal silicon atoms.

6. The composition of claim 1 further including 0.1 to 2 percent byweight of the composition of Fe,O

7. The composition of claim 1 wherein the acrylic ester ismethylacrylate.

8. The composition of claim 1 wherein the acrylic ester istrimethylolpropanetrimethacrylate.

9. The composition of claim 1 wherein the asbestos fiber is selectedfrom anthophyllite, crysotile and crocidolite.

1. A heat vulcanizable silicone rubber composition comprising (a) 15 to94.75 percent by weight of an organopolysiloxane polymer having aviscosity of at least 100,000 centipoise at 25*C of the formula,(R)aSiO4 a/2 (1) (b) 5 to 75 percent by weight of asbestos fiber, and(c) 0.25 to 10.0 percent by weight of an acrylic ester selected from thegroup consisting of
 2. The composition of claim 1 further including asilica filler which comprises 10 to 60 percent by weight of theorganopolysiloxane.
 3. The composition of claim 2 further including 0.1to 8 percent by weight of the organopolysiloxane of a peroxide curingcatalyst.
 4. The composition of claim 1 wherein there is added to theorganopolysiloxane a process aid which comprises 1 to 25 percent byweight of said organopolysiloxane.
 5. The composition of claim 4 whereinthe process aid is a dihydrocarbon-substituted polysiloxane oil having ahydrocarbon substituent to silicon atom ratio of from 1.6 to 2.0 andwhere said hydrocarbon substituents comprise at least one memberselected from the class consisting of methyl, ethyl, vinyl, allyl,cyclohexenyl and phenyl groups, said polysiloxane oil comprisingpolysiloxane molecules containing an average of from one to two loweralkoxy groups bonded to each of the terminal silicon atoms.
 6. Thecomposition of claim 1 further including 0.1 to 2 percent by weight ofthe composition of Fe2O3.
 7. The composition of claim 1 wherein theacrylic ester is methylacrylate.
 8. The composition of claim 1 whereinthe acrylic ester is trimethylolpRopanetrimethacrylate.